1. Field of the Invention
This invention relates generally to a wireless communication system, and more particularly, to a wireless communication system that is capable of effectively suppressing electro-magnetic interference.
2. Description of Related Art
The application scope of wireless communication systems continues to broaden, encompassing such devices as mobile phones, cordless phones, wireless local area networks, wireless headsets and so on.
As shown in FIG. 1, a radio frequency circuit architectural diagram of a prior communication system comprises: a transceiver 10, a band pass filter (BPF) 11, a power amplifier (PA) 12, a low noise amplifier (LNA) 13, a switcher 14, and an antenna 15. In transmitting mode, after a local oscillation frequency signal LO and an intermediate frequency signal IF are mixed inside the transceiver 10 to generate a radio frequency signal RF to be sent, the radio frequency signal RF undergoes first a filter process through the band pass filter 11 for filtering out noise, and then undergoes an amplifying process through the amplifier 12, and subsequently is radiated into air via the antenna 15 after passing through the switcher 14. In receiving mode, a radio frequency signal RF in air is received via the antenna 15, and the received radio frequency is then sent to the low noise amplifier 13 through the switcher 14, and subsequently the received RF signal is processed through the low noise amplifier 13 and then passed to the transceiver 10.
As shown in FIG. 1, the band pass filter 11 is located just before the power amplifier 12 for suppressing the LO and IF signals emitted from the transceiver 10. However, due to design restriction on the oscillator inside the transceiver 10 for generating the LO and IF signals, the band pass filter 11 needs high suppression capability to filter out the LO and IF signals emitted from the transceiver 10. In addition, in the wireless communication system, the response of the band pass filter is affected by external impedance, thereby creating various cut-off effects and thereby being incapable of effectively filtering out the LO and IF signals emitted from the transceiver 10. Consequently, the LO signal and the IF signal traveling along with the RF signal to be radiated are sent into the power amplifier 12 and are amplified and then undesirably radiated via the antenna 15, thus forming electro-magnetic interference (EMI). It further affects wireless communication systems that utilize frequencies corresponding to the LO and IF signals. Accordingly, the wireless communication industry is adopting approaches such as decreasing the emissive power, applying absorbing material, and so on to reduce EMI; however, these prior approaches result in not only higher costs but also greater application difficulties.